Page 140 - Introduction to Mineral Exploration

6: REMOTE SENSING 123

dips appear steeper than they actually are. It (Fig. 6.12b) is a dendritic pattern modified by
is necessary to judge dips and group them structure, such as a well-jointed, flat-lying,
into estimated categories, e.g. 0–5°, 6–10°, massive sandstone. Trellis drainage (Fig. 6.12c)
11–25°, 26–45°, 46–85°, vertical. The vertical has one dominant direction with subsidiary
exaggeration works to the advantage of the drainage at right angles. It occurs in areas
photogeologist, as it may lead to the identifica- of folded sedimentary rocks. Radial drainage
tion of subtle changes in slope in otherwise (Fig. 6.12d) radiates outwards from a central
rounded and subdued topographical features. area, typical of domes and volcanoes. Centrip-
etal drainage (Fig. 6.12e) is the reverse of radial
drainage where drainage is directed inwards. It
Tone
occurs in limestone sinkholes, glacial kettle
Tone refers to the brightness at any point on a holes, volcanic craters, and interior basins (e.g.
panchromatic photograph. Tone is affected by Lake Eyre, Australia and Lake Chad). Deranged
many factors such as: nature of the rock (sand- drainage (Fig. 6.12f) consists of disordered,
stone is light, but shale is dark), light condi- short, aimlessly wandering streams typical of
tions at the time of photographing (cloud, haze, ablation till areas.
sun angle), film, filters, and film processing. These are all destructional drainage patterns.
These effects mean that we are interpret- There are numerous constructional landforms
ing relative tone values. In general terms basic such as alluvial fans, deltas, glacial outwash
extrusive and intrusive igneous rocks (lava, plains, and other superficial deposits. These are
dolerite) have a darker tone, while bedded sand- only indirectly of value in exploration studies
stone, limestone, quartz schists, quartzite, (except for placer and sand and gravel explora-
and acid igneous rocks are generally lighter. tion) and are described in detail by Siegal and
Mudstone, shale, and slate have intermediate Gillespie (1980) and Drury (2001).
tones.
Subtle differences in rock colors are more
readily detected using color photographs, but Drainage texture
these are more expensive. Subtle differences in Drainage texture is described as either coarse
soil moisture and vegetation vigor can be more or fine (Fig. 6.12h). The coarse texture develops
readily detected using color IR, but even these on well-drained soil and rock with little sur-
change with the time of the year. face run off, e.g. limestone, chalk. Fine texture
develops where soils and rocks have poor
internal drainage and high run off, e.g. lava and
Texture
shale.
There is a large variation in apparent texture
of the ground surface as seen on aerial photo-
graphs. Texture is often relative and subjective, Erosion
but some examples are limestone areas which Erosion is a direct extension of the description
may be mottled or speckled, whilst shale is of drainage above, but gullies, etc., often follow
generally smooth, sandstone is blocky, and lines of weakness and thus exaggerate features
granite is rounded. such as joints, fractures, and faults.

Drainage pattern Vegetation and land use
This indicates the bedrock type which in turn The distribution of natural and cultivated
influences soil characteristics and site drainage vegetation often indicates differences in rock
conditions. The six most common drainage pat- type, e.g. sandstone and shale may be cultiv-
terns are: dendritic, rectangular, trellis, radial, ated, while dolerite is left as rough pasture. On
centripetal, deranged (Fig. 6.12). Dendritic the other hand forests may well obscure dif-
drainage (Fig. 6.12a) occurs on relatively ferences so great care must be taken to draw
homogenous material such as flat-lying sedi- meaningful conclusions when annotating areas
mentary rock and granite. Rectangular drainage which have a dense vegetation cover. If an area